I also used to have a whiteboard on the back of my laptop lid. The problem is though that it doesn’t really hold up well if you put the laptop in a case or a backpack. Or if you carry it around it will smear on your arm.

Still I prefer whiteboards to pen & paper because I can make quick corrections and iterations (I do interface & interaction design). That’s why I just ordered a Noteboard: http://www.thenoteboard.com/index.php (I didn’t get it yet though so I don’t know how good it is.)